1. Field of the Invention
The present invention relates to electrodes for solid oxide electrochemical cells and more specifically to a method of fabricating electrodes on solid oxide electrochemical cells by sintering. The electrochemical cells may include fuel cells, electrolyzers and sensors that operate on the basis of electromotive force measurement and/or current measurement and which comprise a solid oxide electrolyte and attached electrodes. Although this invention is primarily directed toward the fabrication of electrodes on fuel cells, it may also be used to fabricate electrodes on a variety of other electrochemical devices.
2. Background Information
Solid oxide fuel cells (SOFCs) are high temperature, electrochemical devices fabricated primarily from oxide ceramics. Typically, they contain an oxygen ion conducting solid electrolyte, such as stabilized zirconia. The electrolyte is usually a thin dense film which separates two porous electrodes comprising an anode and a cathode. The cathode, which is maintained in an oxidizing atmosphere, is usually oxide doped for high electrical conductivity, such as strontium doped lanthanum manganite. The anode, on the other hand, is maintained in a reducing atmosphere and is usually a cermet such as nickel-zirconia. Finally, an interconnection is usually employed which is a dense, electronically conducting oxide material which is stable in both reducing and oxidizing environments, such doped lanthanum chromite. The interconnection is deposited on a cell as a thin gas-tight layer in such a manner that it permits the anodes and cathodes of adjacent cells to be electrically connected in series. The gas-tightness of the interconnection, in combination with that of the electrolyte, insures that the entire cell is gas-tight, preventing mixing of the anode and cathode atmospheres.
Solid oxide cells can be operated in either an electrolysis mode or in a fuel cell mode. In an electrolysis mode, DC electrical power and steam or carbon dioxide or mixtures thereof are supplied to the cell which then decomposes the gas to form hydrogen or carbon monoxide or mixtures thereof, as well as oxygen. In the fuel cell mode, the cell operates by electrochemically oxidizing a gaseous fuel such as hydrogen, carbon monoxide, methane or other fuels to produce electricity and heat.
Nickel plus yttria-stabilized zirconia cermet fuel electrodes have been studied and used for many years as a means to improve thermal expansion match of the anode and electrolyte, and to minimize the effect of oversintering of nickel, which can result in poorer adherence due to a reduction of points of contact of nickel particles with the electrolyte, poorer electrochemical performance, and separation of the anode from the electrolyte during thermal cycles which can occur during fabrication and maintenance procedures. One type of Ni/YSZ cermet used in solid oxide fuel cells is made by coating a porous nickel particle layer covering the electrolyte with a layer of yttria-stabilized zirconia by an electrochemical vapor deposition (EVD) process at elevated temperature. This creates a zirconia skeleton encasing the nickel particles, giving a cermet that adheres tightly to the electrolyte and prevents spalling of the nickel from the electrolyte. This EVD process provides an excellent anode, but it is desirable to fabricate the anode with a less costly process.
The use of nickel-zirconia cermet anodes produced by EVD on solid oxide electrolyte fuel cells is exemplified in U.S. Pat. No. 4,490,444 to Isenberg. which provides an anode that is compatible in chemical, electrical and physical-mechanical characteristics such as thermal expansion with the solid oxide electrolyte to which it is attached. U.S. Pat. No. 4,597,170 to Isenberg addresses bonding and thermal expansion problems between the anode and solid oxide electrolyte by use of a skeletal embedding growth of, for example, ionically conducting zirconia doped with minor amounts of yttria. The skeletal growth extends from the electrolyte/anode interface into a porous nickel layer. U.S. Pat. No. 4,582,766 to Isenberg discloses oxidizing the nickel in a cermet electrode to form a metal oxide layer in order to reduce gas diffusion overvoltages.
Although attempts have been made at producing cermet anodes by low-cost sintering techniques rather than EVD processes, such sintered anodes often deteriorate during operation, which may be related to continued sintering, poor adherence to the electrolyte at useful anode sheet resistance values, and low porosity at suitable resistivity values. Conventional sintering processes are disclosed in U.S. Pat. Nos. 4,971,830 and 5,035,962 to Jensen.
Each of the patents cited above is incorporated herein by reference.
Despite the foregoing efforts, there is still a need for a low cost sintering process for the fabrication of anode structures which eliminates the need for electrochemical vapor deposition, while providing the desired combination of conductivity, adherence, electrochemical performance and stability over a long period of time.